Method and system for additive manufacturing using closed-loop temperature control

ABSTRACT

A system for additive manufacturing comprises a dispensing head for dispensing building materials on a working surface, a hardening system for hardening the building materials, a cooling system for evacuating heat away from the building materials, and a computerized controller. A thermal sensing system is mounted above the working surface in a manner that allows relative motion between the sensing system and the working surface, and is configured to generate sensing signals responsively to thermal energy sensed thereby. The controller controls the dispensing head to dispense the building materials in layers, the sensing system to generate the sensing signals only when the sensing system is above the building materials once hardened, and the heat evacuation rate of the cooling system responsively to the sensing signals.

RELATED APPLICATIONS

This application is a National Phase of PCT Patent Application No.PCT/IL2019/051072 having International filing date of Sep. 27, 2019,which claims the benefit of priority under 35 USC § 119(e) of U.S.Provisional Patent Application No. 62/737,172 filed on Sep. 27, 2018.The contents of the above applications are all incorporated by referenceas if fully set forth herein in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to additivemanufacturing and, more particularly, but not exclusively, to method andsystem for additive manufacturing using closed-loop temperature control.

Additive manufacturing (AM) is a technology enabling fabrication ofarbitrarily shaped structures directly from computer data via additiveformation steps. The basic operation of any AM system consists ofslicing a three-dimensional computer model into thin cross sections,translating the result into two-dimensional position data and feedingthe data to control equipment, which fabricates a three-dimensionalstructure in a layerwise manner.

Additive manufacturing entails many different approaches to the methodof fabrication, including three-dimensional (3D) printing such as 3Dinkjet printing, electron beam melting, stereolithography, selectivelaser sintering, laminated object manufacturing, fused depositionmodeling and others.

Some 3D printing processes, for example, 3D inkjet printing, are beingperformed by a layer by layer inkjet deposition of building materials.Thus, a building material is dispensed from a dispensing head having aset of nozzles to deposit layers on a supporting structure. Depending onthe building material, the layers may then be cured or solidified usinga suitable device.

Various three-dimensional printing techniques exist and are disclosedin, e.g., U.S. Pat. Nos. 6,259,962, 6,569,373, 6,658,314, 6,850,334,6,863,859, 7,183,335, 7,209,797, 7,225,045, 7,300,619, 7,500,846,7,991,498 and 9,031,680 and U.S. Published Application Nos. 20160339643and 20060054039, all by the same Assignee, and being hereby incorporatedby reference in their entirety.

U.S. Published Application No. 20060054039 discloses a printing cell forthree-dimensional printing of modeling material on a tray to form anobject. The printing cell includes a temperature control unit. Thecontrol unit includes a heating source and a cooling source, and isassociated with a temperature sensing unit to sense the temperature ofthe printing cell, the tray, and the building material.

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the present inventionthere is provided a system for additive manufacturing. The systemcomprises: a dispensing head for dispensing building materials on aworking surface; a hardening system for hardening the buildingmaterials; a cooling system for evacuating heat away from the buildingmaterials; a thermal sensing system, mounted above the working surfacein a manner that allows relative motion between the sensing system andthe working surface, and being configured to generate sensing signalsresponsively to thermal energy sensed thereby; and a computerizedcontroller having a circuit for controlling the dispensing heads todispense the building materials in layers, a circuit for controlling thesensing system to generate the sensing signals only when the sensingsystem is above the building materials once hardened, and a circuit forcontrolling a heat evacuation rate of the cooling system responsively tothe sensing signals.

According to some embodiments of the invention the thermal sensingsystem comprises at least one pixelated sensor, wherein the sensingsignals constitutes a thermal map of the hardened building materials.

According to some embodiments of the invention the circuit forcontrolling the heat evacuation rate is configured to identify in thethermal map a first pixel population of a higher temperature and asecond pixel population of a lower temperature, and to control the heatevacuation rate based on the first and the second population.

According to some embodiments of the invention the circuit forcontrolling the heat evacuation rate is configured to control the heatevacuation rate so as to maintain the first pixel population at atemperature that is below a first predetermined threshold.

According to some embodiments of the invention, the circuit forcontrolling the heat evacuation rate is configured to control the heatevacuation rate so as to maintain the second pixel population at atemperature that is above a second predetermined threshold.

According to some embodiments of the invention, the dispensing head ismounted between the hardening system and the sensing system.

According to some embodiments of the invention the dispensing headcomprises a nozzle array having a length and being arranged along anindexing direction, and wherein the sensing system is mounted at adistance from the working surface selected such that a field-of-view ofthe sensing system over the working surface along the indexing directionmatches the length.

According to some embodiments of the invention the circuit forcontrolling the sensing system is configured to adapt a sampling rate ofthe signals to a speed of the relative motion.

According to some embodiments of the invention, the cooling systemcomprises a fan, and wherein the circuit for controlling the heatevacuation rate is configured to vary a rotating speed of the fan.

According to some embodiments of the invention the rotating speed of thefan is varied according to a function of a temperature differencebetween a temperature sensed by the sensing system and a predeterminedtemperature, the function comprises a quadratic function of thetemperature difference.

According to an aspect of some embodiments of the present inventionthere is provided a method of additive manufacturing. The methodcomprises: dispensing building materials on a receiving surface;hardening the building materials to form hardened materials; sensingthermal energy emitted at least by the hardened building materials;evacuating heat away from the building materials responsively to thermalenergy emitted by the hardened building materials, but not responsivelyto thermal energy emitted by other objects; and repeating thedispensing, the hardening, the sensing, and the evacuating a pluralityof times to form a three-dimensional object in layers corresponding toslices of the object.

According to some embodiments of the invention, the sensing is by atleast one pixelated sensor to provide a thermal map of the hardenedbuilding materials.

According to some embodiments of the invention the method comprisesidentifying in the thermal map a first pixel population of a highertemperature and a second pixel population of a lower temperature,wherein the evacuating the heat is at a rate selected based on the firstand the second population.

According to some embodiments of the invention, the rate is selected soas to maintain the first pixel population at a temperature that is belowa first predetermined threshold.

According to some embodiments of the invention rate is selected so as tomaintain the second pixel population at a temperature that is above asecond predetermined threshold.

According to some embodiments of the invention the method comprises thedispensing is by a dispensing head which comprises a nozzle array havinga length and being arranged along an indexing direction, and wherein thesensing is characterized by a field-of-view over the hardened materialsthat matches the length.

According to some embodiments of the invention, the method comprisesadapting a sampling rate of sensing to a speed of the relative motion.

According to some embodiments of the invention, the evacuating is by afan, and the method comprises varying a rotating speed of the fan.

According to some embodiments of the invention the rotating speed of thefan is varied according to a function of a temperature differencebetween a temperature corresponding to the sensed thermal energy and apredetermined temperature, the function comprises a quadratic functionof the temperature difference.

According to an aspect of some embodiments of the present inventionthere is provided a system for additive manufacturing. The systemcomprises: a dispensing head for dispensing building materials; ahardening system for hardening the building materials; a cooling systemhaving a fan for evacuating heat away from the building materials; and acomputerized controller having a circuit for controlling the dispensingheads to dispense the building materials in layers, the hardening systemto harden the building materials, and the cooling system to evacuateheat away from the layers, wherein the circuit is configured to vary arotating speed of the fan as a decreasing function of an area of arespective layer.

According to some embodiments of the invention for at least one of thelayers, the dispensing heads dispense the building materials in morethan one pass over a receiving surface, and wherein the circuit isconfigured to select the rotating speed of the fan based on a number ofthe passes.

According to some embodiments of the invention for at least one of thelayers, the circuit is configured to control the hardening system toharden a portion of the layer following a dispensing of one type ofbuilding material to form the portion but before a dispensing of anothertype of building material to form another portion of the layer.

According to some embodiments of the invention, the circuit isconfigured to access a computer readable medium storing groups ofbuilding materials, to associate a respective building material with oneof the groups, and to select the rotating speed also based on theassociation.

According to some embodiments of the invention the system comprises athermal sensing system configured to sense a temperature of at least oneof the layers, wherein the circuit is configured to receive temperaturesensing signals from the sensing system and to select the rotating speedbased on the temperature sensing signals.

According to an aspect of some embodiments of the present inventionthere is provided a system for additive manufacturing. The systemcomprises: a plurality of dispensing heads for dispensing a respectiveplurality of building materials; a hardening system for hardening thebuilding materials; and a computerized controller having a circuit forcontrolling the dispensing heads to dispense the building materials inlayers, and the hardening system to harden the building materials,wherein for at least one of the layers, the circuit is configured tocontrol the hardening system to harden a first portion of the layerfollowing a dispensing of one type of building material to form thefirst portion, but before a dispensing of another type of buildingmaterial to form a second portion of the layer.

According to some embodiments of the invention, the hardening of thefirst portion is at a first temperature, and wherein the circuit isconfigured to control the hardening system to harden the second portion,at a second temperature, which is different than the first temperature.

According to some embodiments of the invention, the second temperatureis higher than the first temperature.

According to some embodiments of the invention, the circuit isconfigured to access a computer readable medium storing groups ofbuilding materials, to associate a respective building material with oneof the groups, and to select a respective hardening temperature based onthe association.

According to some embodiments of the invention, the circuit isconfigured to transmit pulsed operating signals to the dispensing heads,in a manner that pulses of different widths are transmitted to differentdispensing heads.

According to some embodiments of the invention, the circuit isconfigured to access a computer readable medium storing groups ofbuilding materials, to associate a respective building material with oneof the groups, and to select a respective pulse width based on theassociation.

According to some embodiments of the invention the system comprises aheating system for heating an environment surrounding the layers,wherein the circuit is configured to access a computer readable mediumstoring groups of building materials, to associate a respective buildingmaterial with one of the groups, and to select control of at least oneof a power and operation duration of the heating system based on theassociation.

According to some embodiments of the invention the heating systemcomprises a heating radiation source generating heating radiation.

According to some embodiments of the invention the radiation is infraredradiation.

According to some embodiments of the invention, the circuit isconfigured to synchronize operation of the heating system with operationof the hardening system.

According to some embodiments of the invention the synchronization issuch that the operation of the hardening system is initiated after theoperation of the heating system is terminated.

According to an aspect of some embodiments of the present inventionthere is provided a method of additive manufacturing of an object, themethod comprises: receiving computer object data defining a shape of theobject; and operating the additive manufacturing system described hereinaccording to the computer object data to manufacture the object.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIGS. 1A-D are schematic illustrations of an additive manufacturingsystem according to some embodiments of the invention;

FIGS. 2A-2C are schematic illustrations of printing heads according tosome embodiments of the present invention;

FIGS. 3A-3B are schematic illustrations demonstrating coordinatetransformations according to some embodiments of the present invention;

FIG. 4 is a flowchart diagram of a method suitable for additivemanufacturing according to various exemplary embodiments of the presentinvention;

FIG. 5 is a schematic illustration of a field-of-view of a thermalsensing system, which can be employed in an AM system according to someembodiments of the present invention;

FIG. 6 is a schematic block diagram of circuits, which can be employedby a controller of an AM system according to some embodiments of thepresent invention;

FIG. 7 is a schematic illustration of a structure suitable for holding athermal sensor and a circuit board according to some embodiments of thepresent invention;

FIG. 8 is a feasibility test graph obtained in experiments performedaccording to some embodiments of the present invention;

FIGS. 9A and 9B are graphs showing raw data received from a sensor in anexperiment in which the sensor was moving with respect to a tray duringa fabrication of a prismatic object.

FIGS. 10A and 10B are graphs showing temperatures measured duringexperiments performed according to some embodiments of the presentinvention using a static high-resolution infrared camera mounted tocapture the entire layer;

FIGS. 11A-C are an illustration (FIG. 11A), a thermal map produced by ahigh-resolution infrared camera (FIG. 11B), and a graph (FIG. 11C)describing a fabrication of an object having a top surface of differentheights, obtained experiments performed according to some embodiments ofthe present invention; and

FIG. 12 is a graph showing temperature readings of a sensor inexperiments in which the sensor was placed at different positions.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to additivemanufacturing and, more particularly, but not exclusively, to method andsystem for additive manufacturing using closed-loop temperature control.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

The method and system of the present embodiments manufacturethree-dimensional objects based on computer object data in a layerwisemanner by forming a plurality of layers in a configured patterncorresponding to the shape of the objects. The computer object data canbe in any known format, including, without limitation, a StandardTessellation Language (STL) or a StereoLithography Contour (SLC) format,Virtual Reality Modeling Language (VRML), Additive Manufacturing File(AMF) format, Drawing Exchange Format (DXF), Polygon File Format (PLY)or any other format suitable for Computer-Aided Design (CAD).

The term “object” as used herein refers to a whole object or a partthereof.

Each layer is formed by additive manufacturing apparatus, which scans atwo-dimensional surface and patterns it. While scanning, the apparatusvisits a plurality of target locations on the two-dimensional layer orsurface, and decides, for each target location or a group of targetlocations, whether or not the target location or group of targetlocations is to be occupied by building material formulation, and whichtype of building material formulation is to be delivered thereto. Thedecision is made according to a computer image of the surface.

In preferred embodiments of the present invention, the AM comprisesthree-dimensional printing, more preferably three-dimensional inkjetprinting. In these embodiments a building material formulation isdispensed from a dispensing head having a set of nozzles to depositbuilding material formulation in layers on a supporting structure. TheAM apparatus thus dispenses building material formulation in targetlocations, which are to be occupied and leaves other target locationsvoid. The apparatus typically includes a plurality of dispensing heads,each of which can be configured to dispense a different buildingmaterial formulation. Thus, different target locations can be occupiedby different building material formulations. The types of buildingmaterial formulations can be categorized into two major categories:modeling material formulation and support material formulation. Thesupport material formulation serves as a supporting matrix orconstruction for supporting the object or object parts during thefabrication process and/or other purposes, e.g., providing hollow orporous objects. Support constructions may additionally include modelingmaterial formulation elements, e.g. for further support strength.

The modeling material formulation is generally a composition which isformulated for use in additive manufacturing and which is able to form athree-dimensional object on its own, i.e., without having to be mixed orcombined with any other substance.

Herein throughout, the phrase “uncured building material” collectivelydescribes the materials that are dispensed during the fabricationprocess so as to sequentially form the layers, as described herein. Thisphrase encompasses uncured materials (also referred to herein asbuilding material formulation(s)) dispensed so as to form the printedobject, namely, one or more uncured modeling material formulation(s),and uncured materials dispensed so as to form the support, namelyuncured support material formulations.

Herein, the dispensed materials are also referred to collectively as“material formulations”. The material formulations provide, typicallywhen hardened (unless indicated otherwise), typically hardened uponexposure to a curing condition as defined herein (unless indicatedotherwise), to form a respective material.

Herein throughout, the phrases “cured modeling material” and “hardenedmodeling material”, which are used interchangeably, describe the part ofthe building material that forms a model object, as defined herein, uponexposing the dispensed building material to curing, and followingremoval of the support material. The cured or hardened modeling materialcan be a single hardened material or a mixture of two or more hardenedmaterials, depending on the modeling material formulations used in themethod, as described herein.

Herein throughout, the phrase “modeling material formulation”, which isalso referred to herein interchangeably as “modeling formulation”,describes a part of the uncured building material, which is dispensed soas to form the model object, as described herein. The modelingformulation is an uncured modeling formulation, which, upon exposure toa curing condition, forms the final object or a part thereof.

An uncured building material can comprise one or more modelingformulations, and can be dispensed such that different parts of themodel object are made upon curing different modeling formulations, andhence are made of different cured modeling materials or differentmixtures of cured modeling materials.

Herein throughout, the phrase “hardened support material” is alsoreferred to herein interchangeably as “cured support material” or simplyas “support material” and describes the part of the building materialthat is intended to support the fabricated final object during thefabrication process, and which is removed once the process is completedand a hardened modeling material is obtained.

Herein throughout, the phrase “support material formulation”, which isalso referred to herein interchangeably as “support formulation” orsimply as “formulation”, describes a part of the uncured buildingmaterial which is dispensed so as to form the support material, asdescribed herein. The support material formulation is an uncuredformulation. When a support material formulation is a curableformulation, it forms, upon exposure to a curing condition, a hardenedsupport material.

Support materials, which can be either liquid materials or hardened,typically gel materials, are also referred to herein as sacrificialmaterials, which are removable after layers are dispensed and exposed toa curing energy, to thereby expose the shape of the final object.

Herein and in the art, the term “gel” describes a material, oftenreferred to as a semi-solid material, which comprises athree-dimensional solid network, typically made of fibrous structureschemically or physically linked therebetween, and a liquid phase encagedwithin this network. Gels are typically characterized by a consistencyof a solid (e.g., are non-fluidic), and feature relatively low Tensilestrength, relatively low Shear Modulus, e.g., lower than 100 kPa, and aShear Loss Modulus to Shear Storage modulus (tan delta, G″/G′) valuelower than 1. Gels can be characterized as flowable when subjected to apositive pressure of at least 0.5 bar, preferably at least 1 bar, orhigher, or, alternatively, as non-flowable when subject to a pressurelower than 1 bar or lower than 0.5 bar or of 0.3 bar or lower.

Currently practiced support materials typically comprise a mixture ofcurable and non-curable materials, and are also referred to herein asgel support material.

Currently practiced support materials are typically water miscible, orwater-dispersible or water-soluble.

Herein throughout, the term “water-miscible” describes a material, whichis at least partially dissolvable or dispersible in water, that is, atleast 50% of the molecules move into the water upon mixture. This termencompasses the terms “water-soluble” and “water dispersible”.

Herein throughout, the term “water-soluble” describes a material thatwhen mixed with water in equal volumes or weights, a homogeneoussolution is formed.

Herein throughout, the term “water-dispersible” describes a materialthat forms a homogeneous dispersion when mixed with water in equalvolumes or weights.

Herein throughout, the phrase “dissolution rate” describes a rate atwhich a substance is dissolved in a liquid medium. Dissolution rate canbe determined, in the context of the present embodiments, by the timeneeded to dissolve a certain amount of a support material. The measuredtime is referred to herein as “dissolution time”.

The final three-dimensional object is made of the modeling material or acombination of modeling materials or modeling and support materials ormodification thereof (e.g., following curing). All these operations arewell-known to those skilled in the art of solid freeform fabrication.

In some exemplary embodiments of the invention an object is manufacturedby dispensing one or more different modeling material formulations. Whenmore than one modeling material formulation is used, each materialformulation is optionally and preferably dispensed from a differentarray of nozzles (belonging to the same or distinct dispensing heads) ofthe AM apparatus.

In some embodiments, the dispensing head of the AM apparatus is amulti-channels dispensing head, in which case different modelingmaterial formulations can be dispensed from two or more arrays ofnozzles that are located in the same multi-channels dispensing head. Insome embodiments, arrays of nozzles that dispense different modelingmaterial formulations are located in separate dispensing heads, forexample, a first array of nozzles dispensing a first modeling materialformulation is located in a first dispensing head, and a second array ofnozzles dispensing a second modeling material formulation is located ina second dispensing head.

In some embodiments, an array of nozzles that dispense a modelingmaterial formulation and an array of nozzles that dispense a supportmaterial formulation are both located in the same multi-channelsdispensing head. In some embodiments, an array of nozzles that dispensea modeling material formulation and an array of nozzles that dispense asupport material formulation are located in separate dispensing headheads.

The material formulations are optionally and preferably deposited inlayers during the same pass of the printing heads. The materialformulations and combination of material formulations within the layerare selected according to the desired properties of the object.

A representative and non-limiting example of a system 110 suitable forAM of an object 112 according to some embodiments of the presentinvention is illustrated in FIG. 1A. System 110 comprises an additivemanufacturing apparatus 114 having a dispensing unit 16 which comprisesa plurality of dispensing heads. Each head preferably comprises an arrayof one or more nozzles 122, as illustrated in FIGS. 2A-C describedbelow, through which a liquid building material formulation 124 isdispensed.

Preferably, but not obligatorily, apparatus 114 is a three-dimensionalprinting apparatus, in which case the dispensing heads are printingheads, and the building material formulation is dispensed via inkjettechnology. This need not necessarily be the case, since, for someapplications, it may not be necessary for the additive manufacturingapparatus to employ three-dimensional printing techniques.Representative examples of additive manufacturing apparatus contemplatedaccording to various exemplary embodiments of the present inventioninclude, without limitation, fused deposition modeling apparatus andfused material formulation deposition apparatus.

Each dispensing head is optionally and preferably fed via a buildingmaterial formulation reservoir which may optionally include atemperature control unit (e.g., a temperature sensor and/or a heatingdevice), and a material formulation level sensor. To dispense thebuilding material formulation, a voltage signal is applied to thedispensing heads to selectively deposit droplets of material formulationvia the dispensing head nozzles, for example, as in piezoelectric inkjetprinting technology. The dispensing rate of each head depends on thenumber of nozzles, the type of nozzles and the applied voltage signalrate (frequency). Such dispensing heads are known to those skilled inthe art of solid freeform fabrication.

Preferably, but not obligatorily, the overall number of dispensingnozzles or nozzle arrays is selected such that half of the dispensingnozzles are designated to dispense support material formulation and halfof the dispensing nozzles are designated to dispense modeling materialformulation, i.e. the number of nozzles jetting modeling materialformulations is the same as the number of nozzles jetting supportmaterial formulation. In the representative example of FIG. 1A, fourdispensing heads 16 a, 16 b, 16 c and 16 d are illustrated. Each ofheads 16 a, 16 b, 16 c and 16 d has a nozzle array. In this Example,heads 16 a and 16 b can be designated for modeling materialformulation/s and heads 16 c and 16 d can be designated for supportmaterial formulation. Thus, head 16 a can dispense a first modelingmaterial formulation, head 16 b can dispense a second modeling materialformulation and heads 16 c and 16 d can both dispense support materialformulation. In an alternative embodiment, heads 16 c and 16 d, forexample, may be combined in a single head having two nozzle arrays fordepositing support material formulation. In a further alternativeembodiment any one or more of the dispensing heads may have more thanone nozzle arrays for dispensing more than one material formulation,e.g. two nozzle arrays for dispensing two different modeling materialformulations or a modeling material formulation and a support materialformulation, each formulation via a different array or number ofnozzles.

Yet it is to be understood that it is not intended to limit the scope ofthe present invention and that the number of modeling materialformulation depositing heads (modeling heads) and the number of supportmaterial formulation depositing heads (support heads) may differ.Generally, the number of modeling heads, the number of support heads andthe number of nozzles in each respective head or head array are selectedsuch as to provide a predetermined ratio, a, between the maximaldispensing rate of the support material formulation and the maximaldispensing rate of modeling material formulation. The value of thepredetermined ratio, a, is preferably selected to ensure that in eachformed layer, the height of modeling material formulation equals theheight of support material formulation. Typical values for a are fromabout 0.6 to about 1.5.

As used herein the term “about” refers to ±10%.

For example, for a=1, the overall dispensing rate of support materialformulation is generally the same as the overall dispensing rate of themodeling material formulation when all modeling heads and support headsoperate.

In a preferred embodiment, there are M modeling heads each having marrays of p nozzles, and S support heads each having s arrays of qnozzles such that M×m×p=S×s×q. Each of the M×m modeling arrays and S xssupport arrays can be manufactured as a separate physical unit, whichcan be assembled and disassembled from the group of arrays. In thisembodiment, each such array optionally and preferably comprises atemperature control unit and a material formulation level sensor of itsown, and receives an individually controlled voltage for its operation.

Apparatus 114 can further comprise a solidifying device 324 which caninclude any device configured to emit light, heat or the like that maycause the deposited material formulation to hardened. For example,solidifying device 324 can comprise one or more radiation sources, whichcan be, for example, an ultraviolet or visible or infrared lamp, orother sources of electromagnetic radiation, or electron beam source,depending on the building material formulation being used. In someembodiments of the present invention, solidifying device 324 serves forcuring or solidifying the modeling material formulation.

In some embodiments of the present invention apparatus 114 comprisescooling system 134 such as one or more fans or the like.

The dispensing head and radiation source are preferably mounted in aframe or block 128, which is preferably operative to reciprocally moveover a tray 360, which serves as the working surface. In someembodiments of the present invention the radiation sources are mountedin the block such that they follow in the wake of the dispensing headsto at least partially cure or solidify the material formulations justdispensed by the dispensing heads. Tray 360 is positioned horizontally.According to the common conventions an X-Y-Z Cartesian coordinate systemis selected such that the X-Y plane is parallel to tray 360. Tray 360 ispreferably configured to move vertically (along the Z direction),typically downward. In various exemplary embodiments of the invention,apparatus 114 further comprises one or more leveling devices 132, e.g. aroller 326. Leveling device 326 serves to straighten, level and/orestablish a thickness of the newly formed layer prior to the formationof the successive layer thereon. Leveling device 326 preferablycomprises a waste collection device 136 for collecting the excessmaterial formulation generated during leveling. Waste collection device136 may comprise any mechanism that delivers the material formulation toa waste tank or waste cartridge.

In use, the dispensing heads of unit 16 move in a scanning direction,which is referred to herein as the X direction, and selectively dispensebuilding material formulation in a predetermined configuration in thecourse of their passage over tray 360. The building material formulationtypically comprises one or more types of support material formulationand one or more types of modeling material formulation. The passage ofthe dispensing heads of unit 16 is followed by the curing of themodeling material formulation(s) by radiation source 126. In the reversepassage of the heads, an additional dispensing of building materialformulation may be carried out, according to predeterminedconfiguration. In the forward and/or reverse passages of the dispensingheads, the materials just dispensed may be straightened by levelingdevice 326, which preferably follows the path of the dispensing heads intheir forward and/or reverse movement. Once the dispensing heads returnto their starting point along the X direction, they may move to anotherposition along an indexing direction, referred to herein as the Ydirection, and continue to build the same layer by reciprocal movementalong the X direction. Alternately, the dispensing heads may move in theY direction between forward and reverse movements or after more than oneforward-reverse movement. The series of scans performed by thedispensing heads to complete a single layer is referred to herein as asingle scan cycle.

Once the layer is completed, tray 360 is lowered in the Z direction to apredetermined Z level, according to the desired thickness of the layersubsequently to be printed. The procedure is repeated to formthree-dimensional object 112 in a layerwise manner.

In another embodiment, tray 360 may be displaced in the Z directionbetween forward and reverse passages of the dispensing head of unit 16,within the layer. Such Z displacement is carried out in order to causecontact of the leveling device with the surface in one direction andprevent contact in the other direction.

System 110 optionally and preferably comprises a building materialformulation supply system 330 which comprises the building materialformulation containers or cartridges and supplies a plurality ofbuilding material formulations to fabrication apparatus 114.

A controller 152 controls fabrication apparatus 114 and optionally andpreferably also supply system 330. Controller 152 can be a computerizedcontroller having an electronic circuit and a non-volatile memory mediumreadable by the circuit, wherein the memory medium stores programinstructions which, when read by the circuit, cause the circuit toperform control operations as further detailed below. In someembodiments of the present invention, the electronic circuit ofcontroller 152 is also configured for performing data processingoperations. Controller 152 preferably communicates with a data processor154, which transmits digital data pertaining to fabrication instructionsbased on computer object data, e.g., a CAD configuration represented ona computer readable medium in a form of a Standard Tessellation Language(STL) format or the like. Typically, controller 152 controls the voltageapplied to each dispensing head or nozzle array and the temperature ofthe building material formulation in the respective printing head.

Once the manufacturing data is loaded to controller 152 it can operatewithout user intervention. In some embodiments, controller 152 receivesadditional input from the operator, e.g., using data processor 154 orusing a user interface 116 communicating with unit 152. User interface116 can be of any type known in the art, such as, but not limited to, akeyboard, a touch screen and the like. For example, controller 152 canreceive, as additional input, one or more building material formulationtypes and/or attributes, such as, but not limited to, color,characteristic distortion and/or transition temperature, viscosity,electrical property, magnetic property. Other attributes and groups ofattributes are also contemplated.

Another representative and non-limiting example of a system 10 suitablefor AM of an object according to some embodiments of the presentinvention is illustrated in FIGS. 1B-D. FIGS. 1B-D illustrate a top view(FIG. 1B), a side view (FIG. 1C) and an isometric view (FIG. 1D) ofsystem 10.

In the present embodiments, system 10 comprises a tray 12 and aplurality of inkjet printing heads 16, each having a plurality ofseparated nozzles, and arranged to receive building material formulationfrom supply system 330. Tray 12 can have a shape of a disk or it can beannular. Non-round shapes are also contemplated, provided they can berotated about a vertical axis.

Tray 12 and heads 16 are optionally and preferably mounted such as toallow a relative rotary motion between tray 12 and heads 16. This can beachieved by (i) configuring tray 12 to rotate about a vertical axis 14relative to heads 16, (ii) configuring heads 16 to rotate about verticalaxis 14 relative to tray 12, or (iii) configuring both tray 12 and heads16 to rotate about vertical axis 14 but at different rotation velocities(e.g., rotation at opposite direction). While the embodiments below aredescribed with a particular emphasis to configuration (i) wherein thetray is a rotary tray that is configured to rotate about vertical axis14 relative to heads 16, it is to be understood that the presentapplication contemplates also configurations (ii) and (iii) Any one ofthe embodiments described herein can be adjusted to be applicable to anyof configurations (ii) and (iii), and one of ordinary skills in the art,provided with the details described herein, would know how to make suchadjustment.

In the following description, a direction parallel to tray 12 andpointing outwardly from axis 14 is referred to as the radial directionr, a direction parallel to tray 12 and perpendicular to the radialdirection r is referred to herein as the azimuthal direction φ, and adirection perpendicular to tray 12 is referred to herein is the verticaldirection z.

The term “radial position,” as used herein, refers to a position on orabove tray 12 at a specific distance from axis 14. When the term is usedin connection to a printing head, the term refers to a position of thehead, which is at specific distance from axis 14. When the term is usedin connection to a point on tray 12, the term corresponds to any pointthat belongs to a locus of points that is a circle whose radius is thespecific distance from axis 14 and whose center is at axis 14.

The term “azimuthal position,” as used herein, refers to a position onor above tray 12 at a specific azimuthal angle relative to apredetermined reference point. Thus, radial position refers to any pointthat belongs to a locus of points that is a straight line forming thespecific azimuthal angle relative to the reference point.

The term “vertical position,” as used herein, refers to a position overa plane that intersect the vertical axis 14 at a specific point.

Tray 12 serves as a supporting structure for three-dimensional printing.The working area on which one or objects are printed is typically, butnot necessarily, smaller than the total area of tray 12. In someembodiments of the present invention the working area is annular. Theworking area is shown at 26. In some embodiments of the presentinvention tray 12 rotates continuously in the same direction throughoutthe formation of object, and in some embodiments of the presentinvention tray reverses the direction of rotation at least once (e.g.,in an oscillatory manner) during the formation of the object. Tray 12 isoptionally and preferably removable. Removing tray 12 can be formaintenance of system 10, or, if desired, for replacing the tray beforeprinting a new object. In some embodiments of the present inventionsystem 10 is provided with one or more different replacement trays(e.g., a kit of replacement trays), wherein two or more trays aredesignated for different types of objects (e.g., different weights)different operation modes (e.g., different rotation speeds), etc. Thereplacement of tray 12 can be manual or automatic, as desired. Whenautomatic replacement is employed, system 10 comprises a trayreplacement device 36 configured for removing tray 12 from its positionbelow heads 16 and replacing it by a replacement tray (not shown). Inthe representative illustration of FIG. 1B tray replacement device 36 isillustrated as a drive 38 with a movable arm 40 configured to pull tray12, but other types of tray replacement devices are also contemplated.

Exemplified embodiments for the printing head 16 are illustrated inFIGS. 2A-2C. These embodiments can be employed for any of the AM systemsdescribed above, including, without limitation, system 110 and system10.

FIGS. 2A-B illustrate a printing head 16 with one (FIG. 2A) and two(FIG. 2B) nozzle arrays 22. The nozzles in the array are preferablyaligned linearly, along a straight line. In embodiments in which aparticular printing head has two or more linear nozzle arrays, thenozzle arrays are optionally and preferably can be parallel to eachother.

When a system similar to system 110 is employed, all printing heads 16are optionally and preferably oriented along the indexing direction withtheir positions along the scanning direction being offset to oneanother.

When a system similar to system 10 is employed, all printing heads 16are optionally and preferably oriented radially (parallel to the radialdirection) with their azimuthal positions being offset to one another.Thus, in these embodiments, the nozzle arrays of different printingheads are not parallel to each other but are rather at an angle to eachother, which angle being approximately equal to the azimuthal offsetbetween the respective heads. For example, one head can be orientedradially and positioned at azimuthal position φ₁, and another head canbe oriented radially and positioned at azimuthal position φ₂. In thisexample, the azimuthal offset between the two heads is φ₁-φ₂, and theangle between the linear nozzle arrays of the two heads is also φ₁-φ₂.The particular direction along which a particular head is oriented(radially) is referred to as the “indexing direction” of the head.

In some embodiments, two or more printing heads can be assembled to ablock of printing heads, in which case the printing heads of the blockare typically parallel to each other. A block including several inkjetprinting heads 16 a, 16 b, 16 c is illustrated in FIG. 2C.

In some embodiments, system 10 comprises a support structure 30positioned below heads 16 such that tray 12 is between support structure30 and heads 16. Support structure 30 may serve for preventing orreducing vibrations of tray 12 that may occur while inkjet printingheads 16 operate. In configurations in which printing heads 16 rotateabout axis 14, support structure 30 preferably also rotates such thatsupport structure 30 is always directly below heads 16 (with tray 12between heads 16 and tray 12).

Tray 12 and/or printing heads 16 is optionally and preferably configuredto move along the vertical direction z, parallel to vertical axis 14 soas to vary the vertical distance between tray 12 and printing heads 16.In configurations in which the vertical distance is varied by movingtray 12 along the vertical direction, support structure 30 preferablyalso moves vertically together with tray 12. In configurations in whichthe vertical distance is varied by heads 16 along the verticaldirection, while maintaining the vertical position of tray 12 fixed,support structure 30 is also maintained at a fixed vertical position.

The vertical motion can be established by a vertical drive 28. Once alayer is completed, the vertical distance between tray 12 and heads 16can be increased (e.g., tray 12 is lowered relative to heads 16) by apredetermined vertical step, according to the desired thickness of thelayer subsequently to be printed. The procedure is repeated to form athree-dimensional object in a layerwise manner.

The operation of inkjet printing heads 16 and optionally and preferablyalso of one or more other components of system 10, e.g., the motion oftray 12, are controlled by a controller 20. The controller can has anelectronic circuit and a non-volatile memory medium readable by thecircuit, wherein the memory medium stores program instructions which,when read by the circuit, cause the circuit to perform controloperations as further detailed below. In some embodiments of the presentinvention, the electronic circuit of controller 20 is also configuredfor performing data processing operations.

Controller 20 can also communicate with a host computer 24 whichtransmits digital data pertaining to fabrication instructions based oncomputer object data, e.g., in a form of a Standard TessellationLanguage (STL) or a StereoLithography Contour (SLC) format, VirtualReality Modeling Language (VRML), Additive Manufacturing File (AMF)format, Drawing Exchange Format (DXF), Polygon File Format (PLY) or anyother format suitable for Computer-Aided Design (CAD). The object dataformats are typically structured according to a Cartesian system ofcoordinates. In these cases, computer 24 preferably executes a procedurefor transforming the coordinates of each slice in the computer objectdata from a Cartesian system of coordinates into a polar system ofcoordinates. Computer 24 optionally and preferably transmits thefabrication instructions in terms of the transformed system ofcoordinates. Alternatively, computer 24 can transmit the fabricationinstructions in terms of the original system of coordinates as providedby the computer object data, in which case the transformation ofcoordinates is executed by the circuit of controller 20.

The transformation of coordinates allows three-dimensional printing overa rotating tray. In conventional three-dimensional printing, theprinting heads reciprocally move above a stationary tray along straightlines. In such conventional systems, the printing resolution is the sameat any point over the tray, provided the dispensing rates of the headsare uniform. Unlike conventional three-dimensional printing, not all thenozzles of the head points cover the same distance over tray 12 duringat the same time. The transformation of coordinates is optionally andpreferably executed so as to ensure equal amounts of excess materialformulation at different radial positions. Representative examples ofcoordinate transformations according to some embodiments of the presentinvention are provided in FIGS. 3A-B, showing a slice of an object (eachslice corresponds to fabrication instructions of a different layer ofthe objects), where FIG. 3A illustrates the slice in a Cartesian systemof coordinates and FIG. 3B illustrates the same slice following anapplication of a transformation of coordinates procedure to therespective slice.

Typically, controller 20 controls the voltage applied to the respectivecomponent of the system 10 based on the fabrication instructions andbased on the stored program instructions as described below.

Generally, controller 20 controls printing heads 16 to dispense, duringthe rotation of tray 12, droplets of building material formulation inlayers, such as to print a three-dimensional object on tray 12.

System 10 optionally and preferably comprises hardening device 324,which may include one, or more radiation sources 18, such as, but notlimited to, an ultraviolet or visible or infrared lamp, or other sourcesof electromagnetic radiation, or electron beam source, depending on themodeling material formulation being used. Radiation source can includeany type of radiation emitting device, including, without limitation,light emitting diode (LED), digital light processing (DLP) system,resistive lamp and the like. Radiation source 18 serves for curing orsolidifying the modeling material formulation. In various exemplaryembodiments of the invention the operation of radiation source 18 iscontrolled by controller 20, which may activate and deactivate radiationsource 18 and may optionally also control the amount of radiationgenerated by radiation source 18.

In some embodiments of the invention, system 10 further comprises one ormore leveling devices 32, which can be manufactured as a roller or ablade. Leveling device 32 serves to straighten the newly dispensedmaterials prior to the dispensing of additional materials thereon. Insome embodiments, leveling device 32 has the shape of a conical rollerpositioned such that its symmetry axis 34 is tilted relative to thesurface of tray 12 and its surface is parallel to the surface of thetray. This embodiment is illustrated in the side view of system 10 (FIG.1C).

The conical roller can have the shape of a cone or a conical frustum.

The opening angle of the conical roller is preferably selected such thatis a constant ratio between the radius of the cone at any location alongits axis 34 and the distance between that location and axis 14. Thisembodiment allows roller 32 to efficiently level the layers, since whilethe roller rotates, any point p on the surface of the roller has alinear velocity which is proportional (e.g., the same) to the linearvelocity of the tray at a point vertically beneath point p. In someembodiments, the roller has a shape of a conical frustum having a heighth, a radius R₁ at its closest distance from axis 14, and a radius R₂ atits farthest distance from axis 14, wherein the parameters h, R₁ and R₂satisfy the relation R₁/R₂=(R−h)/h and wherein R is the farthestdistance of the roller from axis 14 (for example, R can be the radius oftray 12).

The operation of leveling device 32 is optionally and preferablycontrolled by controller 20 which may activate and deactivate levelingdevice 32 and may optionally also control its position along a verticaldirection (parallel to axis 14) and/or a radial direction (parallel totray 12 and pointing toward or away from axis 14.

In some embodiments of the present invention system 10 comprises coolingsystem 134 (see FIGS. 1C and 1D) such as one or more fans or the like.

In some embodiments of the present invention printing heads 16 areconfigured to reciprocally move relative to tray along the radialdirection r. These embodiments are useful when the lengths of the nozzlearrays 22 of heads 16 are shorter than the width along the radialdirection of the working area 26 on tray 12. The motion of heads 16along the radial direction is optionally and preferably controlled bycontroller 20.

Some embodiments contemplate the fabrication of an object by dispensingdifferent material formulations from different dispensing heads. Theseembodiments provide, inter alia, the ability to select materialformulations from a given number of material formulations and definedesired combinations of the selected material formulations and theirproperties. According to the present embodiments, the spatial locationsof the deposition of each material formulation with the layer isdefined, either to effect occupation of different three-dimensionalspatial locations by different material formulations, or to effectoccupation of substantially the same three-dimensional location oradjacent three-dimensional locations by two or more different materialformulations so as to allow post deposition spatial combination of thematerial formulations within the layer, thereby to form a compositematerial formulation at the respective location or locations.

Any post deposition combination or mix of modeling material formulationsis contemplated. For example, once a certain material formulation isdispensed it may preserve its original properties. However, when it isdispensed simultaneously with another modeling material formulation orother dispensed material formulations which are dispensed at the same ornearby locations, a composite material formulation having a differentproperty or properties to the dispensed material formulations is formed.

The present embodiments thus enable the deposition of a broad range ofmaterial formulation combinations, and the fabrication of an objectwhich may consist of multiple different combinations of materialformulations, in different parts of the object, according to theproperties desired to characterize each part of the object.

Further details on the principles and operations of an AM systemsuitable for the present embodiments are found in U.S. Pat. No.9,031,680, the contents of which are hereby incorporated by reference.

The inventors of the present invention have devised a technique thatallows maintaining within the AM system a temperature that is within apredetermined range of temperatures. This is advantageous since itallows the system of the present embodiments to adapt the workingtemperature of the AM process based on information provided by theoperator or extracted automatically.

One type of information can relate to the building material or materialsto be used for the AM. Since the optimal working temperatures may differamong different types of building materials, the system of the presentembodiments can adapt the working temperature based on the material ormaterials to be dispensed. Thus, in some embodiments of the presentinvention the predetermined range of temperatures is selected by theoperator or automatically by the controller of the AM system, based onthe materials that are to be dispensed. For example, when the materialsare acrylic, relatively low temperatures (e.g., from about 60° to about100°) are preferred, when the materials are solvent-based materials(e.g., ceramic materials, polyimide-containing materials) that requiresolvent drying, more elevated temperatures (e.g., from about 100° toabout 150°) are preferred, and when the martials requite sintering (forexample, electrically conductive materials) even higher temperatures(e.g., from about 150° to about 200°) are preferred.

Some modeling materials, particularly UV polymerizable materials,exhibit undesired deformation such as curling during the fabrication ofthe object. Such curling tendency was found to be the result of materialshrinkage during phase transition from liquid to solid. The extent ofcurling is a measurable quantity. A suitable process for measuring theextent of curling can include fabrication of an object of apredetermined shape, e.g., an elongated bar having a rectangular orsquare cross section, on a flat and horizontal surface, applying apredetermined load on one end of the object, and measuring the elevationof the opposite end above the surface.

The extent of curling correlates with the existence of a temperaturegradient in the manufactured object along the vertical direction, andalso with the difference between the characteristic Heat DeflectionTemperature (HDT) of the building material and the temperature withinthe AM system during fabrication.

As used herein, the term “Heat Deflection Temperature” (HDT) refers to atemperature at which the respective material or combination of materialsdeforms under a predetermined load at some certain temperature. Suitabletest procedures for determining the HDT of a material or combination ofmaterials are the ASTM D-648 series, particularly the ASTM D-648-06 andASTM D-648-07 methods.

Without wishing to be bound to any theory, it is postulated that atemperature within the system during fabrication which is close to theHDT that the materials develop during curing, allows the materials toundergo stress relaxation or plastic deformation to a larger extentcompared to a situation in which the temperature within the system isfar from the HDT.

In various exemplary embodiments of the invention the predeterminedrange of temperatures is a range that encompasses the HDT of thematerials to be dispensed by the system with a tolerance of, ±10° morepreferably ±5°. This is advantageous from the standpoint of reducing thelikelihood for curling, since it reduces the difference between thedeveloped HDT and the temperature within the AM system.

FIG. 4 is a flowchart diagram of a method suitable for additivemanufacturing according to various exemplary embodiments of the presentinvention. It is to be understood that, unless otherwise defined, theoperations described hereinbelow can be executed eithercontemporaneously or sequentially in many combinations or orders ofexecution. Specifically, the ordering of the flowchart diagrams is notto be considered as limiting. For example, two or more operations,appearing in the following description or in the flowchart diagrams in aparticular order, can be executed in a different order (e.g., a reverseorder) or substantially contemporaneously. Additionally, severaloperations described below are optional and may not be executed.

The method can be executed by an AM system (e.g., system 110 or system10) operated by a controller (e.g., controller 152 or 20). The methodbegins at 400 and optionally and preferably proceeds to 401 at which atwhich computer object data that collectively pertain to athree-dimensional shape of the object are received. The data can bereceived by a data processor (e.g., processor 154 or 24) operativelyassociated with the AM system. For example, the data processor canaccess a computer-readable storage medium (not shown) and retrieve thedata from the medium. The data processor can also generate the data, ora portion thereof, instead of, or in addition to, retrieving data fromthe storage medium, for example, by means of a computer aided design(CAD) or computer aided manufacturing (CAM) software. The computerobject data typically include a plurality of slice data each defining alayer of the object to be manufactured. The data processor can transferthe data, or a portion thereof, to the controller of the AM system.Typically, but not necessarily, the controller receives the data on aslice-by-slice basis. The data can be in any data format known in theart, including, any of the aforementioned computer object data formats.

The method continues to 402 at which building materials are dispensed,for example, using one or more of dispensing heads 16, on a receivingsurface. The receiving surface can be the working surface of the AMsystem (e.g., tray 12 or 360) or it can be a previously formed layer ofone or more building materials. The method continues to 403 at which thedispensed building materials are hardened to form hardened materials.Operation 403 can be executed by hardening device 324, and may includeapplying curing radiation to the dispensed material(s). The type ofradiation (e.g., electromagnetic, electron beam, etc.) is selected basedon the building materials being used. For example, for UV polymerizablematerials an ultraviolet electromagnetic radiation is preferred.

The method optionally and preferably proceeds to 404 at which thermalenergy emitted at least by the hardened building materials is sensed.The sensing is by a thermal sensing system schematically shown in FIGS.1A and 1B at 138. Sensing system 138 generates sensing signalsresponsively to thermal energy sensed by sensor 138. Sensing system 138typically comprises one or more thermal sensors, such as, but notlimited to, one or more infrared sensors.

Preferably, sensing system 138 is mounted above the working surface(e.g., tray 12 or 360) of the AM system in a manner that allows relativemotion between sensing system 138 and the working surface along thescanning direction x or the azimuthal direction φ. This allowsconfiguring sensing system 138 to sense, locally and optionally andpreferably also selectively, the energy that is emitted from a portionof the topmost layer of the object during fabrication, wherein therelative motion between sensing system 138 ensures that sensing system138 scans the topmost layer in a serial manner to locally sense thethermal energy emitted by the layer section by section. Thus, inpreferred embodiments of them invention, sensing system 138 generates atime-series of sensing signals, each signal containing local informationpertaining to thermal energy emitted by a different section of thetopmost layer.

Preferably, one or more of the sensors of sensing system 138 is/aremounted at a sufficient distance from hardening device 324 so as toreduce or prevent thermal energy generated by device 324 from arrivingdirectly to the respective sensor. For example, as illustrated in FIG.1A, sensing system 138 can be mounted on frame 128 opposite to device324, such that head 16 is between sensing system 138 and device 324. Inembodiments in which device 324 generates less heat or provide a moredirectional radiation (e.g., when device 324 comprises one or more LEDs)the sensing system may be mounted closer to or adjacent to the device324, since in these embodiments there is less thermal cross talk betweendevice 324 and system 138.

In various exemplary embodiments of the invention sensing system 138comprises at least one pixelated sensor in which case the sensingsignals generated by the respective sensor constitute a thermal map ofthe hardened building materials.

The advantage of the aforementioned configuration in which the sensingsystem 138 scans the topmost layer is that it allows using low-costsensors to obtain thermal data at sufficiently high spatial resolution.For example, suppose that it is desired to obtain thermal data atminimal feature size of ΔA mm², and that the surface area of aparticular layer is A mm². In this case it is required to have A/ΔA datasamples to provide the desired resolution. It is appreciated that forsmall value of ΔA and large value A, the ratio A/ΔA can be rather large(e.g., 10,000 or more). A high resolution camera that provides a largenumber of data samples momentarily can be rather expensive and bulky.According to the present embodiments, sensing system 138 does not needto provide all A/ΔA data samples during a signal capture. By scanningthe topmost layer and providing a time-series of sensing signals, thesame resolution can be achieved by pixelated sensors in which the numberof pixels is significantly less than A/ΔA. In various exemplaryembodiments of the invention the number of pixels of each pixelatedsensor of sensing system 138 is less than 1,000, more preferably lessthan 500, more preferably less than 250, more preferably less than 125,e.g., 100 or less. For example, in experiments performed by the presentinventors it was found that the aforementioned allows achieving anadequate spatial resolution using a pixelated sensor in which thesensing elements are arranged over a 4×16 array.

The angular field-of-view of sensing system 138 along the indexing andscanning direction, respectively, is preferably from about 40°×10° toabout 120°×25°, more preferably from about 40°×10° to about 60°×15°.When sensing system 138 comprises one or more pixelated sensors, eachsensor is preferably mounted at a distance selected such that thespatial resolution of the pixelated sensor corresponds to a minimalidentifiable feature size of about 1×1 mm, more preferably 0.8×0.8 mm,more preferably 0.6×0.6 mm, more preferably 0.5×0.5 mm.

It is expected that during the life of a patent maturing from thisapplication many relevant thermal sensors will be developed and thescope of the term thermal sensor is intended to include all such newtechnologies a priori.

Sensing system 138 is provided with a field-of-view over the hardenedmaterials, allowing sensing system 138 to sense thermal energy over anarea of the hardened materials. In some embodiments of the presentinvention the field-of-view along the indexing direction matches thelengths of the nozzle array(s) of the dispensing head along the indexingdirection. These embodiments are illustrated in FIG. 5, showing sensingsystem 138, head 16 and tray 12 or 360. While FIG. 5 shows a sensingsystem having a single sensor, it is to be understood thatconfigurations in which sensing system 138 comprises a plurality ofsensors is also contemplated. The lengths of the nozzle array (not shownin FIG. 5) of head 16 along the indexing direction (along direction y,in case of system 110, and along direction r in case of system 10) isdenoted by L_(H) and is defined as the distance along the indexingdirection over which the nozzles (not shown in FIG. 5) are distributed.The field-of-view sensing system 138 along the indexing direction isdenoted by L_(FOV). Note that L_(FOV) is the combined field-of-view ofsensing system 138. When sensing system 138 comprises more than onesensor, the field-of-view of each individual sensor can be less thanL_(FOV). As illustrated, L_(H)=L_(FOV). Thus, when sensing system 138comprises a single sensor, the field-of-view of the signal sensorpreferably equals or approximately equals L_(H), and when sensing system138 comprises a plurality of sensors, the field-of-view of each sensorcan be less than L_(H), but the combined field-of-view of all thesensors preferably equals or approximately equals L_(H). For example,when there are n sensors in sensing system 138, the field-of-view ofeach individual sensor can be at least L_(H)/n and less than L_(H).Typically, the field-of-view of each individual sensor is close to butlarger than L_(H)/n to allow overlaps between field-of-views of adjacentsensors.

The field-of-view provided to sensing system 138 along the orthogonalhorizontal direction (the x or φ direction) need not to be the same asthe width of the head along this direction, since preferably there isrelative motion between the sensor and the tray along direction x or φ.

Returning to FIG. 4, the method proceeds to 405 at which heat isevacuated away from the building materials. This can be executed byoperating cooling system 134. Preferably, but not necessarily, coolingsystem 134 is mounted above the working surface (e.g., tray 12 or 360)of the AM system in a manner that allows relative motion between coolingsystem 134 and the working surface along the scanning direction x or theazimuthal direction φ. This allows cooling system 134 to evacuate theheat in a generally localized manner, wherein heat is evacuated moreefficiently from regions below cooling system 134 than from regionsfarther from cooling system 134. During the relative motion, differentregions are effectively cooled by system 134. In some embodiments of theinvention, cooling system 134 is mounted on the printing block of theprinting system together with sensing system 138 and one or moredispensing heads 16. For example, as illustrated in FIG. 1A, coolingsystem 134, sensing system 138 and dispensing heads 16 are mounted onframe 128.

The heat is evacuated at a rate that is selected responsively to thethermal energy emitted by the hardened building materials. The rate canbe selected by controlling the power supplied to cooling system 134. Forexample, when cooling system 134 comprises one or more fans, the poweris controlled to vary the rotation speed of the fan(s), hence also theheat evacuation rate. Control over the power supplied to cooling system134 can be effected by varying any operational parameter used by system134, including, without limitation, voltage, pulse width, pulserepetition rate, pulse width modulation, and the like.

In various exemplary embodiments of the invention rate that is selectedbased only on the thermal energy emitted by the hardened buildingmaterials. Specifically, in these embodiments the rate of heatevacuation is not selected based on thermal energy emitted by otherobjects (e.g., building materials before being hardened, or othercomponents of the system, including, without limitation, the tray or theside walls of the system). This can be ensured by operating the thermalsensor in synchronization with the dispensing heads and the motion ofthe tray and/or the heads. The synchronization can such that the sensingsignals are sampled only after the heads dispense the material and onlywhen the hardened material enters the field-of-view of the sensor. Suchsynchronization can be executed by controller 20 or 152 based oninformation pertaining to the state of the dispensing head and thelocation of the dispending head above the tray.

As a representative example for a synchronized operation, consider theconfiguration illustrated in FIG. 1A. In the exemplified configuration,the heads 16 are between the hardening device 324 and the sensing system138. Consider a dispensing protocol in which the heads dispense whilemoving in the +x direction (to the left, in the notation of FIG. 1A),wherein during the motion in the opposite direction −x (to the right, inthe notation of FIG. 1A), there is no dispensing. Suppose that thefield-of-view of sensing system 138 along the y direction is asillustrated in FIG. 5, namely the same as the length of the nozzle arrayof head 16. During the motion in the +x direction, the head 16 dispensesbuilding materials that are leveled by the leveling device 132 andhardened by hardening device 324. During the motion in the oppositedirection, no material is dispensed, and so the uppermost layer ofobject 112 is hardened when sensing system 138 moves above it. When theuppermost layer enters the field-of-view of sensing system 138 whilemoving in the −x direction, controller 152 signals sensing system 138 tobegin the sampling of the sensing signal, and when the uppermost layerexits the field-of-of sensing system 138 controller 152 signals sensingsystem 138 to stop the sampling. This ensures that the sensing signalscontain only information pertaining to the thermal energy emitted by thehardened materials forming object 112.

As another example similar synchronization for a synchronized operation,consider a rotary system in which the tray rotates in one direction, butnot in the opposite direction. For example, consider the configurationillustrated in FIG. 1B, in which the tray 12 rotates along the +φdirection. In this configuration, the material enters the field-of-viewof sensing system 138 after passing below hardening device 324, and istherefore hardened. When the uppermost layer of the fabricated objectenters the field-of-view of sensing system 138, controller 20 signalssensing system 138 to begin the sampling of the sensing signal, and whenthe uppermost layer exits the field-of-of sensing system 138 controller20 signals sensing system 138 to stop the sampling.

Aside for the timing of the sampling to ensure that sampling occurs onlyduring the time at which hardened materials are within the field-of-viewof sensing system 138, the present embodiments also contemplatesynchronizing the sampling rate of sensing system 138 with the speed ofthe relative motion between the sensor and the tray. This isadvantageous since it reduces fluctuations in the accuracy of thesensing. Preferably, sampling rate increases linearly with the speed ofthe motion.

The heat evacuation rate is preferably varied dynamically in close loopwith the signals from the thermal sensor to ensure that temperature ofthe hardened materials, preferably the hardened materials at the topmostlayers of the fabricated object, are within a predetermined range oftemperatures. This can be done by thresholding. Typically, when thesensed temperature is above a first predetermined threshold T₁, thepower supplied to the cooling system 134 is increased, to ensure ahigher heat evacuation rate so that the temperature does notsignificantly increased beyond T₁. Preferably, the power supplied to thecooling system 134 is decreased when the sensed temperature is below asecond predetermined threshold T₂ (T₂<T₁), the power supplied to thecooling system 134 is increased, to reduce the heat evacuation rate sothat the temperature does not significantly decrease below T₂.

In embodiments in which sensing system 138 provides a thermal map thethermal map is preferably analyzed to identify in the map a first pixelpopulation of a higher temperature, and a second pixel population of alower temperature. When the temperature of the first pixel population isabove the threshold T₁, the power supplied to the cooling system 134 canbe increased. Preferably, when the temperature of the second pixelpopulation is below the threshold T₂, the power supplied to the coolingsystem 134 can be decreased.

In some embodiments of the present invention the evacuation rate isvaried as a nonlinear function of the temperature difference ΔT betweenthe sensed temperature and a predetermined reference temperature.Preferably, the nonlinear function comprises a quadratic function of ΔT.

In some embodiments of the present invention the controller of the AMsystem controls the hardening device to harden one type of buildingmaterial before dispensing another type of building material. Theseembodiments are particularly useful when the preferred fabricationconditions (e.g., preferred temperature ranges) of two or more of thematerials used during the AM substantially differ from each other.Consider, for example, a fabrication process in which two types ofmaterials are used, wherein the preferred temperature ranges for thesetwo materials do not overlap. Since the temperature ranges do notoverlap, the operational parameters required for operating the coolingsystem 134 in order to provide heat evacuation rates that maintain thesetemperature ranges may also differ.

Suppose that the computer object data for a particular slice is suchthat both materials are to be dispensed to form the same layer. In thiscase, one of the materials is dispensed and hardened to form a firstportion of the layer, and the power of the cooling system is controlledto ensure that the temperature at the vicinity of the first portion ofthe layer is within the preferred range of temperatures for thismaterial. Thereafter, the other material is dispensed and hardened toform a second portion of the layer, and the power of the cooling systemis controlled to ensure that the temperature at the vicinity of thesecond portion of the layer is within the preferred range oftemperatures for the other material. Such a procedure can ensure thatboth materials enjoy the preferred range of temperatures even when theranges do not overlap.

The temperature ranges and/or operational parameters for the coolingsystem can be provided to AM system manually by the operator, or theycan be obtained automatically. For example, a computer readable mediumstoring groups of building materials and corresponding operationalparameters can be accessed. Each of the materials to be used in the AMprocess can be associated with one of the groups, and the correspondingoperational parameters can be extracted.

The present embodiments additionally contemplated a procedure in whichthe rate of heat evacuation is selected based, at least in part, on thecomputer object data received at 401. In these embodiments, operation404 can be skipped. Alternatively, the rate of heat evacuation can beselected based on both the sensing 404 and the computer object data.

In a representative exemplified embodiment, the rate of heat evacuationis varied as a decreasing function of an area of a respective layerdispensed by the dispensing head. Since larger area of hardened materialallows higher heat dissipation, varying the rate of heat evacuation as adecreasing function of the area can reduce the likelihood that thetemperature at the layer significantly exceeds the temperature thresholdT₁.

In another representative exemplified embodiment, the rate of heatevacuation is selected based on the number of passes that the dispensinghead performs over the receiving surface per layer. Since the number ofpasses correlates with the area of the layer, the rate of heatevacuation can be varied as a decreasing function of the number ofpasses.

From 405 the method optionally and preferably loops back to 401 or 402to form an additional layer of the object. The loop can continue untilall the layers of the object are formed. The method ends at 406,

As stated, the rate of heat evacuation is preferably controlled by thecontroller of the AM system. Typically the controller includes one ormore circuits for performing the various operations. Although thecontrollers 152 and 20 are shown in FIGS. 1A and 1B as single blocks, itis to be understood that the various circuits the perform the operationscan be physically attached to different components of the AM system.

A schematic block diagram of circuits which can be employed by thecontroller 20/152 according to some embodiments of the present inventionare illustrated in FIG. 6. In the illustrated example, which is not tobe considered as limiting, three circuit boards 600, 602, and 604 areshown. Circuit board 600 serves as the main control of the AM system,and is optionally and preferably configured to control variouscomponents of the AM system other than the thermal sensing system 138and the cooling system 134, in response to input data received fromprocessor 154 or 24. Thus, for example, circuit board 600 can beconfigured to control at least the printing heads, and the relativemotion between the heads and the tray.

Circuit board 602 serves as the controller of sensing system 138(“Sensor Control”) and is configured to activate and deactivate samplingof the signals received from the sensor, and optionally and preferablyalso to control the sampling rate, responsively to control signals fromcircuit board 600 (“Main Control”). Sampled signals are transmitted byboard 602 to processor 24 or 154, for processing. Alternatively, thesampled signals can be transmitted to main control board 600. Processor24/154 or main control board 600 analyzes the signal to determinewhether or not the heat evacuation rate is to be varied as furtherdetailed hereinabove and transmits signals pertaining to the analysis tocircuit board 604.

Circuit board 604 serves as a “cooling control” and is configured tocontrol the operation of the cooling system based on the analysisperformed by processor 154 or 24, e.g., by varying the operationparameter as further detailed hereinabove. Board 604 can optionally andpreferably receive feedback signals from system 134. The feedbacksignals contain information pertaining to the operation of system 134.For example, when system 134 comprises a fan, the feedback signals cancontain tachometer data describing the rotation of the fan.

Circuit boards 600, 602, and 604 can all be arranged on the samephysical board. Alternatively, one or more of boards 600, 602, and 604can be physically separated from the others. In a preferredimplementation, boards 600, 602 are arranged on the same physical board,and board 604 is separated from boards 600 and 602. In this embodiment,board 604 can be mounted on the same structure with sensing system 138.A representative and non-limiting illustration of a structure 610suitable for holding one or more of the sensors of sensing system 138and board 604 is shown in FIG. 7. Also shown in FIG. 7 is thefield-of-view L_(FOV) of sensing system 138 along the indexingdirection.

As used herein the term “about” refers to ±10%.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration.” Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

The word “optionally” is used herein to mean “is provided in someembodiments and not provided in other embodiments.” Any particularembodiment of the invention may include a plurality of “optional”features unless such features conflict.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range. Whenever a numerical range isindicated herein, it is meant to include any cited numeral (fractionalor integral) within the indicated range. The phrases “ranging/rangesbetween” a first indicate number and a second indicate number and“ranging/ranges from” a first indicate number “to” a second indicatenumber are used herein interchangeably and are meant to include thefirst and second indicated numbers and all the fractional and integralnumerals therebetween.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Various embodiments and aspects of the present invention as delineatedhereinabove and as claimed in the claims section below find experimentalsupport in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions illustrate some embodiments of the invention in anon-limiting fashion.

Example 1 Prototype System

Experiments have been conducted to investigate the ability of aprototype AM system having a thermal sensor movable relative to the traycan be stabilize the temperature of the hardened building material. Inall the experiments described the cooling system included fans and theAM system was a system as schematically illustrated in FIG. 1A.

FIG. 8 is a feasibility test graph showing the temperature of the layerin degrees Celsius as a function of the time in seconds, in anexperiment in which dispensing in 300 DPI in the indexing direction wasemployed. The temperatures were measured using a static high-resolutioninfrared camera mounted to capture the entire layer. Shown is thetemperature when the fans were constantly off (open circles), constantlyon (crosses), and controlled by the controller (plus symbols). Thecontroller was set to maintain a temperature of 75° C. As shown, thecontroller successfully stabilized the temperature.

FIG. 9A is a graph showing raw data (dark grey line) and filtered data(white line) received from sensing system 138, in an experiment in whichsensing system 138 included a single sensor moving with respect to thetray during fabrication of a prismatic object. Shown is the temperaturewithin the field-of-view of the sensor in degrees Celsius as a functionof the time in minutes. The temperatures were measured using an infraredcamera mounted to capture the entire layer. Shown is the temperaturewhen the fans were controlled by the controller (diamonds). Thecontroller was set to maintain a temperature of 75° C. As shown, thecontroller successfully stabilized the temperature.

FIG. 9B is a graph showing raw data received from sensing system 138, inan experiment in which sensing system 138 included a single sensormoving with respect to the tray during fabrication of a prismaticobject. Shown is the temperature within the field-of-view of the sensorin degrees Celsius as a function of the time in minutes. Thetemperatures were measured using an infrared camera mounted to capturethe entire layer. Shown is the temperature when the fans were controlledby the controller (diamonds). The controller was set to maintain atemperature of 72° C. As shown, the controller successfully stabilizedthe temperature.

FIGS. 10A and 10B are graphs showing temperatures measured using astatic high-resolution infrared camera mounted to capture the entirelayer. The data correspond to experiments in which the controller wasset to maintain a temperature of 75° C. (FIG. 10A) and 72° C. (FIG.10B). As shown, the controller successfully prevents the temperaturefrom significantly rising above the input temperature.

FIGS. 11A-C are an illustration, a thermal map produced by ahigh-resolution infrared camera, and a graph describing a fabrication ofan object having a top surface of different heights. The controller wasset to maintain a temperature of less than 80° C.

Four top surfaces are marked on the illustration (FIG. 11A), each havinga different high. The highest surface is marked “part 1”, the next tohighest surface is marked “part 2”, the lowest surface is marked “part3”, and the next to lowest surface is marked “part 4”. Four pixelpopulations are marked on the thermal map (FIG. 11B), population Nos. 1through 4, respectively corresponding to part Nos. 1 through 4. Thehighest temperature is of population 1, since it is the newly formedsurface. The graph (FIG. 11C) shows the temperature of each surface indegrees Celsius as a function of the time in minutes. Square symbolscorrespond to the temperatures of population 1, open triangle symbolscorrespond to the temperatures of population 2, cross symbols correspondto the temperatures of population 3, and solid circle symbols correspondto the temperatures of population 4. The controller was successfullyable to identify the population with the highest temperature and tomaintain its temperature at about 80° C.

FIG. 12 is a graph showing the temperature readings of sensing system138 in degrees Celsius as a function of the time in seconds, in anexperiment in which sensing system 138 was placed between head 16 andhardening device 324 (upper curve), and the temperature readings ofsensing system 138 in an experiment in which sensor 138 when head 16 wasbetween sensing system 138 and hardening device 324 (lower curve),demonstrating that reading errors are significantly reduced when thesensor is farther from the hardening device. In both experiments,sensing system 138 included a single sensor. In this example, thehardening system included a mercury lamp that generated high temperaturecross-talk with the sensing system. It is envisioned that the use of LEDor cooler hardening system, may reduce the crosstalk, in which case thesensing system may be mounted closer to or adjacent to the hardeningsystem.

Example 2 Additional Consideration

This example describes a computerized controller that facilitatesadditive manufacturing (e.g., inkjet printing) of objects from buildingmaterials that require different manufacturing conditions. Some of thematerials are based on solvents (e.g., ceramic, conductive and PI-basedinks), other materials, such as, but not limited to, foamable, support,and standard materials (e.g., the Vero™ family of materials) are 100%reactive formulations. The solvent-based materials have differentsolvents (e.g., Hexyl Acetate, TGME) and different amount of solvents(e.g., 20-35 wt. %). The computerized controller of this example allowsa different drying process for different solvent-based materials, forexample, using an IR lamp. Other materials, e.g., electricallyconductive inks, are sintered in addition to the drying operation, andthe computerized controller of this example also facilitates thesintering operation. The fully reactive formulations, such as, but notlimited to, support materials and Vero™ family of materials, are heatsensitive.

It was found by the inventors that the manufacturing conditions that aresuitable for different types of materials do not match, and may evencontradict each other.

Since different building material may have different rheologicalproperties, the computerized controller of this example allowsdispensing different building materials at different jetting conditions,wherein the jetting conditions include, without limitation, jettingtemperature, pulse shape, and pulse width. Optionally and preferably thecomputerized controller of this example allows applying differenthardening (e.g., curing), drying and/or sintering conditions fordifferent building materials.

In some embodiments of the present invention, a computer readable mediumcan include a list of building materials that are classified into groupsof materials. An example list of building materials is provided in Table1, below.

TABLE 1 Process Material temperature Remarks Acrylic material  60 C.-100C. At higher temperature the (Vero, Agilus ™, jetted material may startSupport) to evaporate and the weight loss becomes significant Materialswhich 100 C.-150 C. A fast-drying process may require solvent dryingcause pitting and cracks in the printed layer Sintering 150 C.-200 C. Ahigher temperature may damage the other model materials

The AM system of the present embodiments can optionally and preferablycomprise a heating system, such as, but not limited to, an IR lamp, andthe computerized controller optionally and preferably controls theheating system, in terms of at least one of the power and exposure time.The heating system can have several functionalities: (1) to heat thesubstrate to the required set point prior to dispensing, and (2) to heatthe dispensed materials for curing, drying and/or sintering. The heatingsystem can be enacted by the hardening device of the AM system, or itcan be provided in addition to the hardening device.

In some embodiments of the present invention, ultraviolet radiation isused for curing photoreactive components. For example, the ultravioletradiation can be applied by LEDs. When the building material is solventbased, the computerized controller of this example optionally andpreferably ensures that drying is completed before curing. For example,the computerized controller of this example can synchronize between theactivation and deactivation of the ultraviolet radiation, the activationand deactivation of the heating system, and the activation anddeactivation of the dispensing heads.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting. In addition, any priority document(s) of this applicationis/are hereby incorporated herein by reference in its/their entirety.

What is claimed is:
 1. A system for additive manufacturing, comprising:a dispensing head for dispensing building materials on a workingsurface; a hardening system for hardening said building materials; acooling system for evacuating heat away from said building materials; athermal sensing system, mounted above said working surface in a mannerthat allows relative motion between said sensing system and said workingsurface, and being configured to generate sensing signals responsivelyto thermal energy sensed thereby; and a computerized controller having acircuit for controlling said dispensing head to dispense said buildingmaterials in layers, a circuit for controlling said sensing system togenerate said sensing signals only when said sensing system is abovesaid building materials once hardened, and a circuit for controlling aheat evacuation rate of said cooling system responsively to said sensingsignals.
 2. The system according to claim 1, wherein said thermalsensing system comprises at least one pixelated sensor, wherein saidsensing signals constitutes a thermal map of said hardened buildingmaterials.
 3. The system according to claim 2, wherein said circuit forcontrolling said heat evacuation rate is configured to identify in saidthermal map a first pixel population of a higher temperature and asecond pixel population of a lower temperature, and to control said heatevacuation rate based on said first and said second population.
 4. Thesystem according to claim 3, wherein said circuit for controlling saidheat evacuation rate is configured to control said heat evacuation rateso as to maintain said first pixel population at a temperature that isbelow a first predetermined threshold.
 5. The system according to claim4, wherein said circuit for controlling said heat evacuation rate isconfigured to control said heat evacuation rate so as to maintain saidsecond pixel population at a temperature that is above a secondpredetermined threshold.
 6. The system according to claim 1, whereinsaid dispensing head is mounted between said hardening system and saidsensing system.
 7. The system according to claim 1, wherein saiddispensing head comprises a nozzle array having a length and beingarranged along an indexing direction, and wherein said sensing system ismounted at a distance from said working surface selected such that afield-of-view of said sensing system over said working surface alongsaid indexing direction matches said length.
 8. The system according toclaim 1, wherein said circuit for controlling said sensing system isconfigured to adapt a sampling rate of said signals to a speed of saidrelative motion.
 9. The system according to claim 1, wherein saidcooling system comprises a fan, and wherein said circuit for controllingsaid heat evacuation rate is configured to vary a rotating speed of saidfan.
 10. The system according to claim 9, wherein said rotating speed ofsaid fan is varied according to a function of a temperature differencebetween a temperature sensed by said sensing system and a predeterminedtemperature, said function comprises a quadratic function of saidtemperature difference.
 11. A method of additive manufacturing,comprising: dispensing building materials on a receiving surface;hardening said building materials to form hardened materials; sensingthermal energy emitted at least by said hardened building materials;evacuating heat away from said building materials responsively tothermal energy emitted by said hardened building materials, but notresponsively to thermal energy emitted by other objects; and repeatingsaid dispensing, said hardening, said sensing, and said evacuating aplurality of times to form a three-dimensional object in layerscorresponding to slices of said object.
 12. The method according toclaim 11, wherein said sensing is by at least one pixelated sensor toprovide a thermal map of said hardened building materials.
 13. Themethod according to claim 12, comprising identifying in said thermal mapa first pixel population of a higher temperature and a second pixelpopulation of a lower temperature, wherein said evacuating said heat isat a rate selected based on said first and said second population. 14.The method according to claim 13, wherein said rate is selected so as tomaintain said first pixel population at a temperature that is below afirst predetermined threshold.
 15. The method according to claim 14,wherein rate is selected so as to maintain said second pixel populationat a temperature that is above a second predetermined threshold.
 16. Themethod according to claim 11, wherein said dispensing is by a dispensinghead which comprises a nozzle array having a length and being arrangedalong an indexing direction, and wherein said sensing is characterizedby a field-of-view over said hardened materials that matches saidlength.
 17. The method according to claim 11, comprising adapting asampling rate of sensing to a speed of said relative motion.
 18. Themethod according to claim 11, wherein said evacuating is by a fan, andthe method comprises varying a rotating speed of said fan.
 19. Themethod according to claim 18, wherein said rotating speed of said fan isvaried according to a function of a temperature difference between atemperature corresponding to said sensed thermal energy and apredetermined temperature, said function comprises a quadratic functionof said temperature difference.